1. Field of the Invention
The present invention relates to protein kinase inhibitors, particularly inhibitors of Raf-kinase. The invention also provides pharmaceutical compositions comprising the compounds of the invention and methods of using the compositions in the treatment of various diseases.
2. Background of the Invention
Protein kinases constitute a large family of structurally related enzymes that effect the transfer of a phosphate group from a nucleoside triphosphate to a Ser, Thr or Tyr residue on a protein acceptor. A vast array of cellular functions, including DNA replication, cell cycle progression, energy metabolism, and cell growth and differentiation, are regulated by reversible protein phosphorylation events mediated by protein kinases. Additionally, protein kinase activity has been implicated in a number of disease states, including cancers. Of the >100 dominant oncogenes known to date, many encode receptor and cytoplasmic tyrosine kinases known to be mutated and/or over expressed in human cancers (Blume-Jensen and Hunter, Nature, 411:355-365 (2001)). Accordingly, protein kinase targets have attracted substantial drug discovery efforts in recent years, with several protein kinase inhibitors achieving regulatory approval (reviewed in Fischer, Curr. Med. Chem., 11:1563 (2004); Dancey and Sausville, Nature Rev. Drug Disc., 2:296 (2003)).
Intracellular signaling pathways activated in response to growth factor/cytokine stimulation are known to control functions such as proliferation, differentiation and cell death (Chiloeches and Marais, In Targets for Cancer Therapy; Transcription Factors and Other Nuclear Proteins, 179-206 (La Thangue and Bandara, eds., Totowa, Humana Press 2002)). One example is the Ras-Raf-MEK-ERK pathway which is controlled by receptor tyrosine kinase activation. Activation of Ras proteins at the cell membrane leads to phosphorylation and recruitment of accessory factors and Raf which is then activated by phosphorylation. Activation of Raf leads to downstream activation of MEK and ERK. ERK has several cytoplasmic and nuclear substrates, including ELK and Ets-family transcription factor, which regulates genes involved in cell growth, survival and migration (Marais et al., J. Biol. Chem., 272:4378-4383 (1997); Peyssonnaux and Eychene, Biol. Cell, 93-53-62 (2001)). As a result, this pathway is an important mediator of tumor cell proliferation and angiogenesis. For instance, overexpression of constitutively active B-Raf can induce an oncogenic event in untransformed cells (Wellbrock et al., Cancer Res., 64:2338-2342 (2004)). Aberrant activation of the pathway, such as by activating Ras and/or Raf mutations, is known to be associated with a malignant phenotype in a variety of tumor types (Bos, Hematol. Pathol., 2:55-63 (1988); Downward, Nature Rev. Cancer, 3:11-22 (2003); Karasarides et al., Oncogene, 23:6292-6298 (2004); Tuveson, Cancer Cell, 4:95-98 (2003); Bos, Cancer Res, 49:4682-4689 (1989)). Activating mutations in B-Raf are found in 60-70% of melanomas. Melanoma cells that carry mutated B-Raf-V599E are transformed, and cell growth, ERK signaling and cell viability are dependent on mutant B-Raf function (Karasarides et al., Oncogene, 23:6292-6298 (2004)).
There are three Raf isoforms, A-Raf, B-Raf and C-Raf (Raf-1), all of which can act as downstream effectors of Ras. Although they show significant sequence similarities, they also exhibit distinct roles in development, in addition to significant biochemical and functional differences. In particular, the high basal kinase activity of B-Raf may explain why mutated forms of only this isoform have been found in human cancers. Nevertheless, the isoforms show redundant functions in facilitating oncogenic Ras-induced activation of the MEK-ERK signaling cascade (Wellbrock, Cancer Res, 64:2338-2342 (2004)). In addition to Raf signaling via the MEK-ERK pathway there is some evidence that C-Raf (and possibly B-Raf and A-Raf) may signal via alternative pathways directly involved in cell survival by interaction with the BH3 family of anti-apoptotic proteins (Wellbrock et al., Nature Rev.: Mol. Cell Biol. 5:875 (2004)).
Inhibitors of the Raf kinases may be expected to interrupt the Ras-Raf signaling cascade and thereby provide new methods for the treatment of proliferative disorders, such as cancer. There is thus a need for new inhibitors of Raf kinase activity.